Known image-forming apparatus have a photoreceptor drum or photoreceptor belt (hereinafter referred to as photoreceptor) as a latent image holding member rotatably supported in the main body of the image-forming apparatus. When the apparatus are operated for image formation, an electrostatic latent image is formed in the photosensitive layer of the photoreceptor and this latent image is made visible with a developer by a developing device and then transferred to a recording medium by corona transfer or with a transfer roller, transfer drum, or transfer belt (hereinafter referred to as intermediate transfer medium).
In full-color image-forming apparatus, two or more photoreceptors and developing devices are used to successively transfer images of two or more colors to a recording medium, e.g., paper, on an intermediate transfer medium or the photoreceptors so as to be superposed thereon and the images transferred are fixed. The apparatus operating in this manner are known as tandem machines. Also known is an intermediate transfer system in which color images are successively transferred firstly to an intermediate transfer medium and the images thus transferred are secondly transferred en bloc to a receiving material.
(1) A cleaner-less system is known in which toner residues remaining on the photoreceptors are removed simultaneously with development.
Furthermore, a technique for improving transfer efficiency is known. In this technique, photoreceptors and a transfer medium are rotated or circulated at different speeds to thereby improve toner releasability, resulting in an increased transfer efficiency. In development with a one-component toner, the toner supplied onto a development roller is spread with a regulation blade so as to form a thin film thereon as evenly as possible in order to impart sufficient frictional charges to the toner. The toner is thus negatively charged with the surface of the development roller and the surface of an edge of the regulation blade.
For the case of using an intermediate transfer medium, a technique for preventing the intermediate transfer medium from being fouled by toners has been proposed. Specifically, it has been proposed to use a toner and transfer medium wherein the work function of the toner ΦT and the work function of the surface of the transfer medium ΦR satisfy the relationship |ΦT−ΦR|≦4.0 (eV) to thereby prevent the transfer medium surface from being fouled by the toner (see, for example, patent document 1). Furthermore, a technique for improving transfer efficiency has been proposed which employs an intermediate transfer medium having such surface properties that it has a contact angle with water of 70° or larger and is more positive in frictional electrification rank than a toner. Due to such surface properties, the intermediate transfer medium has enhanced toner releasability and the Coulomb force exerted between the intermediate transfer medium and the toner is reduced, whereby a satisfactory image free from blind spots can be obtained (see, for example, patent document 2).
However, when used for development with toners of different colors and transfer of the color images, those techniques have been insufficient in preventing color mixing of the toners.
On the other hand, particle size reduction in toners has had a drawback that since it reduces toner flowability, electrification by friction with the development roller surface and with the regulation blade becomes difficult and, as a result, sufficient charges cannot be imparted. Because of this, the toner comes to have a charge amount distribution. It is unavoidable that even a toner for negative electrification gives a positively charged toner. As a result, fog occurs in nonimage areas on the photoreceptor.
A technique for diminishing fog is known which comprises using an elevated regulation pressure in development with a nonmagnetic one-component toner. However, toner overcharge occurs and this tends to result in a reduced toner concentration in development or a reduced transfer efficiency. A technique for overcoming this problem is known in which the amount of a toner attached to the development roller after regulation is adjusted to a value within a proper range (i.e., w/ρ is from 0.2 to 0.8, wherein w is the toner coat amount (mg) per cm2 of the toner support surface and ρ is the true density of the toner (g/cm3)) (see, for example, patent document 3, patent document 4, and patent document 5). However, it has been difficult to prevent fog and reverse toner transfer.
A method of full-color image formation has been proposed in which toners having a small particle diameter are used and the maximum amount of each of the toners of respective colors to be deposited on a receiving material is regulated to 5.0 g/m2 or smaller in order to improve electrification characteristics and to reduce graininess for image quality improvement (see, for example, patent document 6). However, this technique was found to be insufficient in the prevention of reverse toner transfer although effective in improving suitability for low-temperature fixing in which toners are thermally fixed evenly.
Furthermore, a method of full-color image formation has been proposed which comprises forming electrostatic latent images on image holding members (photoreceptors), developing the latent images with charged color toners of yellow, magenta, and cyan and with a black toner, transferring the developed images to an intermediate transfer medium having an electrical resistance of from 108 to 1012 Ω·cm, subsequently superposing the image developed with the black toner on the intermediate transfer medium to conduct first transfer, and then transferring the toner images to another transfer medium to conduct second transfer (see, for example, patent document 7).
There is a description in that patent document to the effect that the intermediate transfer medium is not electrified by repetitions of the first transfer and the transfer efficiency of the black toner, which is finally deposited for development and subjected to first transfer, is increased. However, this prior-art technique is insufficient in the prevention of reverse toner transfer.
An apparatus for color image formation has also been proposed in which a black toner is first deposited for development and color toners of yellow, magenta, and cyan are deposited thereafter for development to thereby prevent the black toner from undergoing color mixing with any of the other color toners and enable the black toner only to be recycled (see, for example, patent document 8). However, this apparatus has been insufficient in the prevention of reverse toner transfer.
Another apparatus for color image formation has been proposed. In this apparatus, toner images are formed on both sides of a receiving material through an intermediate transfer medium, and color toner images of yellow, magenta, cyan, and black are superposed in such sequence that cyan and black are transferred first and last, respectively, and yellow and magenta are transferred between these (see, for example, patent document 9). However, this apparatus has been insufficient in the prevention of reverse toner transfer with respect to each of the toner layers.
It has been proposed to use a constant-voltage power source for a first-transfer part and a constant-current power source for a second-transfer part (see, for example, patent document 10). However, this technique also has been insufficient in the transfer efficiency of toner layers and in the prevention of reverse toner transfer.
A tandem image-forming apparatus of the toner recycle type having two or more image holding members and two or more developing devices is known in which toners are recovered by cleaning from the image holding members and returned to the developing devices for respective colors (see, for example, patent document 11 and patent document 12). However, this apparatus has been insufficient because considerable fog occurs and the amount of toners transferred reversely is large.
An image-forming apparatus likewise employing a tandem mechanism has been proposed in which development and cleaning are simultaneously conducted in each developing device (see, for example, patent document 13 and patent document 14). This technique enables size reduction in image-forming apparatus. However, the proposed apparatus has been still insufficient in increasing the transfer efficiency to thereby prevent image fog and reverse toner transfer.
It has further been proposed to use spherical toners to conduct non-contact development and thereby eliminate the necessity of a cleaner (see, for example, patent document 15).
In this proposed technique, toners having a roundness of 0.96 or higher are used to realize a high transfer efficiency, and the toners remaining in a slight amount on the photoreceptors are first recovered with holding rollers and thereafter transferred to an intermediate transfer medium to conduct cleaning. However, since the holding rollers are used for preventing toner color mixing, this technique is disadvantageous from the standpoint of reducing the sizes of members to be disposed around the photoreceptors. The image-forming apparatus according to this technique hence has a large width.
Furthermore, a technique has been proposed in which a spherical toner comprising a combination of a toner having a roundness of from 0.950 to 0.995 with silica, alumina, and titania is used in combination with magnetic brush development to simultaneously conduct the development and cleaning in a development part (see, for example, patent document 16). However, this technique has failed to prevent reverse toner transfer.
(2) In the process for color image formation in which toner images are transferred to an intermediate transfer medium, subsequently transferred en bloc to a recording medium such as paper, and then fixed thereon, there have been troubles that a transfer failure occurs to form a vermiculate image and that toner scattering occurs, resulting in poor image reproducibility. A technique for eliminating these troubles has been proposed in which toners are deposited for development in ascending order of charge amount (see, for example, patent document 17).
A technique for forming images with satisfactory color reproducibility has been proposed in which a transfer voltage is selected so that the toner image to be formed as the lowermost layer among toner images successively formed on an intermediate transfer medium can be transferred at an increased transfer efficiency (see, for example, patent document 18).
With respect to an image-forming apparatus in which a receiving material bearing a color image on each side is processed to fix the toner images en bloc, it has been proposed to form toner images on an intermediate transfer medium in such sequence that cyan and black are deposited first and last, respectively, and yellow and magenta are deposited therebetween (see, for example, patent document 9). Furthermore, a technique has been proposed in which when toners of three colors, i.e., yellow, cyan, and magenta, are superposed to form an image on an intermediate transfer medium, that one of the cyan and magenta toners which is deposited earlier contains a larger amount of a flowability-imparting agent and the absolute value of toner charge amount is increased to thereby obtain an image free from defects caused by transfer failures, such as toner scattering, blind spots, uneven image surfaces, and fog (see, for example, patent document 19).
However, none of the techniques described above has succeeded in sufficiently eliminating failures in transfer from the intermediate transfer medium, etc.
A technique for eliminating transfer failures occurring in transfer from an intermediate transfer medium to a recording medium at the nip between these has been proposed. In this technique, a transfer roller to which a transfer bias is applied is disposed on the back side of the intermediate transfer belt, to which toner images are to be transferred from the photoreceptor, in a position located downstream from and close to the region where the intermediate transfer belt is in contact with the photoreceptor (see, for example, patent document 20). However, this technique has been insufficient in the efficiency of transfer of images of superposed toners of three colors.
Furthermore, a method of image formation with an intermediate transfer medium has been proposed in which a constant-voltage power source and a constant-current power source are used for a first-transfer part and a second-transfer part, respectively, to thereby stabilize the efficiency of transfer of toner images of all colors (see, for example, patent documents 21 and 10). However, this technique has been still insufficient in increasing the efficiency of transfer of images made up of superposed toners of three colors.
On the other hand, intermediate transfer media are made of, e.g., a conductive rubber composition having a given volume resistivity. Too low volume resistivities result in current leakage and in problems concerning images formation, such as paper soils. On the other hand, use of intermediate transfer media having a volume resistivity exceeding a given value results in a poor transfer efficiency and, hence, such transfer media are unsuitable for practical use.
Conductive belts formed from a conductive rubber composition obtained by incorporating a carbon black as an electronic-conductivity-imparting agent into a base material such as a rubber or plastic have been used as intermediate transfer media.
However, such intermediate transfer media, in which electrical conductivity is regulated by the addition of an electronic-conductivity-imparting agent, have had a problem that even a slight change in the amount of the electronic-conductivity-imparting agent or uneven distribution of the electronic-conductivity-imparting agent results in considerable unevenness of electrical resistance, unstable electronic conductivity changing with time, etc.
Furthermore, larger amounts of the electronic-conductivity-imparting agent added result in an increased dependence of electrical resistance to applied voltage, and this has posed a problem that a precise device for controlling applied voltage is necessary for obtaining a constant electrical resistance and a problem that the resultant rubber compositions have impaired processability. It has hence been proposed to add an ionic-conductivity-imparting agent to an ion-conductive polymer or rubber to thereby regulate the volume resistivity of the rubber or polymer to a value in a given range.
Moreover, an intermediate transfer medium having a sea-island structure comprising an ion-conductive polymer as a discontinuous phase and a polymer with reduced moisture permeability as a continuous phase has been proposed as a transfer medium which has reduced unevenness of electrical resistance and in which the electrical resistance is stable under fluctuating environmental conditions (see, for example, patent document 22). However, this intermediate transfer medium has been ineffective in sufficiently improving the transferability of images of superposed toners.
Patent Document 1: JP-A-3-62072
Patent Document 2: JP-A-9-230714
Patent Document 3: JP-A-6-194943
Patent Document 4: JP-A-9-62030
Patent Document 5: JP-A-11-218957
Patent Document 6: JP-A-2002-131973
Patent Document 7: JP-A-8-248779
Patent Document 8: JP-A-2000-206755
Patent Document 9: JP-A-2002-31933
Patent Document 10: JP-A-2002-116599
Patent Document 11: JP-A-2001-092208
Patent Document 12: JP-A-2002-174934
Patent Document 13: JP-A-5-53482
Patent Document 14: JP-A-8-146652
Patent Document 15: JP-A-11-249452
Patent Document 16: JP-A-2000-075541
Patent Document 17: JP-A-10-207164
Patent Document 18: JP-A-5-27548
Patent Document 19: JP-A-2002-278159
Patent Document 20: JP-A-9-152791
Patent Document 21: JP-A-2002-49190
Patent Document 22: JP-A-11-181311